The present invention relates in general to chemical strengthening of glass substrates. More particularly, the present invention relates to a method of adjusting the pH of a strengthening melt for use in strengthening glass substrates, such as glass disk substrates for use in data storage devices.
A typical data storage device includes a medium for storing data, typically in magnetic, magneto-optical or optical form, and a transducer used to write and read data respectively to and from the medium. A disk drive data storage device, for example, includes one or more data storage disks coaxially mounted on a hub of a spindle motor. The spindle motor rotates the data storage disks at speeds typically on the order of several thousand or more revolutions-per-minute. Digital information, representing various types of data, is typically written to and read from the data storage disks by one or more transducers, or read/write heads, which are mounted to an actuator assembly and passed over the surface of the rapidly rotating disks.
In a typical magnetic disk drive, for example, data is stored on a magnetic layer coated on a disk substrate. Several characteristics of disk substrates significantly affect the areal density of a disk drive. One such characteristic that significantly affects the areal density of a disk drive is the uniformity of the surface of the disk substrate, i.e., the absence of substrate surface defects. It is generally recognized that minimizing the flyheight, i.e., the clearance distance between the read/write head and the surface of a data storage disk, generally provides for increased areal densities. It is also recognized in the art, however, that the smoothness of the surface of a data storage disk becomes a critical factor and design constraint when attempting to minimize the flyheight. A significant decrease in flyheight provided by the use of data storage disks having highly uniform recording surfaces can advantageously result in increased transducer readback sensitivity and increased areal density of the disk drive. The uniformity of disk substrate surfaces affects the uniformity of the recording surfaces because the layers sputtered onto the disk substrate, such as the magnetic layer, replicate any irregular surface morphology of the disk substrate.
Conventionally, disk substrates have been based upon aluminum, such as NiP coated Al/Mg alloy substrates. Coating the aluminum magnesium alloy with a nickel-phosphorus plate provides a harder exterior surface which allows the disk substrate to be polished and superfinished. Typically, the Al/Mgxe2x80x94NiP substrate is superfinished to a smooth finish with a colloidal slurry, e.g., a pH adjusted aqueous slurry containing colloidal silica and/or colloidal alumina particles and an etching agent such as aluminum nitrate, prior to sputtering with thin film magnetic coatings. The colloidal slurry is then cleaned from the substrate by the general cleaning mechanisms of mechanical scrubbing, dispersion and etching.
After cleaning, the substrates are sputtered with a series of layers, e.g., a chrome underlayer, a magnetic layer and a carbon protection layer. If residual slurry particles are left on the substrate or there is galling to the relatively soft NiP layer, the sputtered layers replicate the irregular surface morphology, creating a bumpy surface on the finished disk. When the read/write head glides over the surface, it crashes into bumps created by the residual particles and/or damage that is higher than the glide clearance. This is known as a glide defect, which can ultimately cause disk drive failure. These bumps further cause magnetic defects, corrosion and decreased disk life. Thus, the residual slurry particles and/or damage needs to be removed from the polished substrate surface so that the substrate is as smooth as possible.
Unfortunately, aluminum-based substrates have relatively low specific stiffness, as well as relatively low impact and dent resistance. For example, the relatively low specific stiffness of the Al/Mgxe2x80x94NiP substrates (typically 3.8 Mpsi/gm/cc) makes this type of disk substrate susceptible to environmental forces which create disk flutter and vibration and which may cause the read/write head to impact and dent the disk substrate surface.
More recently, glass substrates have been used for disk drives in portable devices, such as laptop computers. Glass substrates have a higher impact and dent resistance than aluminum-based substrates, which is important in portable devices where the unit is subject to being bumped, dropped and banged around, causing the read/write head to bang on the disk substrate surface. Moreover, the specific stiffness of glass or glass-ceramic substrates (typically xe2x89xa66 or 7 Mpsi/gm/cc) is typically higher than that of aluminum-based substrates. As discussed in more detail below, glass substrates are typically strengthened by immersion in a strengthening melt. In the strengthening melt, an ion exchange process strengthens the glass substrate by exchanging smaller ions near the substrate surface for larger ions of the strengthening melt below the transformation temperature of the glass to generate pressure stress zones at the substrate surface.
An additional benefit of glass is that it is easier to polish to and maintain as a smooth surface finish (as compared to NiP) than aluminum-based substrates. A smoother substrate allows the read/write head to fly closer to the disk, which produces a higher density recording. Glide height for some computer disk drives is on the order of 20 nanometers (about 200 xc3x85) and less, which is an extremely small interface distance. Thus, the fact that glass substrates can be polished to smoother finishes makes an industry shift from Al-based substrates to glass substrates desirable, not only for disk drives used in portable devices, but for disk drives used in stationary devices as well.
Just as with aluminum-based substrates, the surface of the glass substrate needs to be polished and superfinished with a slurry to provide an atomically smooth surface. Typically, the glass substrate is superfinished to a smooth finish with a colloidal slurry, e.g., a pH adjusted aqueous slurry containing colloidal silica and/or colloidal alumina particles and an etching agent such as cerium sulfate, prior to strengthening in a strengthening melt and sputtering with thin film magnetic coatings. In this superfinishing polish process, slurry particles attach to the surface being polished. Just as with aluminum-based substrates, if these particles are left in place on the glass substrate, glide defects occur that can ultimately cause disk drive failure. These glide defects further cause magnetic defects, corrosion and decreased disk life. Typically, polyvinyl alcohol (PVA) pad scrubbing, ultrasonics or megasonics are used to remove the slurry particles from the glass substrate. In addition, acid or base solutions may be used to etch the glass substrate or undercut the slurry particles.
However, even after the glass substrate has been successfully superfinished and cleaned, the surface uniformity of the glass substrate is not assured because subsequent immersion of the glass substrate in the strengthening melt can also present a surface uniformity problem. This problem can be especially troublesome with respect to low glide heights (typically xe2x89xa620 nanometers) and near contact recording. Strengthening melts, which are typically nitrates such as potassium nitrate and/or sodium nitrate, are subject to pH shift that can cause glass substrates strengthened therein to etch, creating angstrom size pits on the surface of the glass substrates. The pH shift can come from sources such as the thermal decomposition of the strengthening melt, the glass substrates themselves (typically, alkali glass), and/or incoming salts with high pH. Typically, the pH shift worsens with repeated use of the strengthening melt to treat more and more glass substrates. One conventional solution to the pH shift problem is to bubble sulfur dioxide gas through the strengthening melt. Unfortunately, the inventors have found that the sulfur dioxide gas solution is not effective because of particle formation, i.e., particles (e.g., sodium sulfite) fall out of the strengthening melt and contaminate the surface of the glass substrate. Another conventional solution to the pH shift problem is to add silicic acid (SiO2.nH2O) as diatomaceous earth to the strengthening melt. Like the sulfur dioxide gas solution, however, the inventors have found the diatomaceous earth/silicic acid solution is not effective because particles (e.g., sodium meta silicate) fall out of the strengthening melt and contaminate the surface of the glass substrate. The particle fall out and contamination problem associated with both these conventional solutions to the pH shift problem, i.e., the sulfer dioxide gas solution and the diatomaceous earth/silicic acid solution, has not been recognized in the art.
If the market trend toward glass substrates in disk drives is to succeed, an enhanced mechanism is required for controlling the pH shift in a strengthening melt for use in strengthening glass substrates. Preferably, such an enhanced mechanism would reduce pitting due by etching (as compared to not controlling the pH shift) but would not cause particle formation that can contaminate the surface of the glass substrate.
An object of the present invention is to provide an enhanced mechanism for controlling the pH shift in a strengthening melt for use in strengthening glass substrates.
Another object of the present invention is to provide such an enhanced mechanism that reduces pitting due by etching (as compared to not controlling the pH shift) and does not cause particle formation that can contaminate the surface of the glass substrate.
These and other objects of the present invention are achieved by a method of adjusting the pH of a strengthening melt for use in strengthening glass substrates, e.g., glass disk substrates for use in data storage devices. A non-particle-forming acid is added to the strengthening melt to lower the pH of the strengthening melt to xe2x89xa68. The acid is added while the strengthening melt is in a molten state and selected to avoid particle formation. Nitric acid, for example, is non-particle-forming with respect to nitrate based strengthening melts such as potassium nitrate and/or sodium nitrate. A base, e.g., sodium hydroxide, may be added if the pH of the strengthening melt falls below 5. Strengthening melts are subject to pH shift that can cause glass substrates strengthened therein to etch, creating pits on the substrate surface. Glass disk substrates treated in the pH adjusted strengthening melt are essentially free from such pits, as well as contamination caused by particle formation.